We have developed an in vitro system to induce immunoglobulin secretion by malignant B cells (lymphomas and leukemias). The immunoglobulin secreted by these cells (IgM) is purified on an affinity column developed in our laboratory. This highly purified monoclonal immunoglobulin will be used for the immunization of mice and subsequent development of monoclonal (hybridoma) antibodies specific to idiotypic determinants associated with the malignant cells. This antibody will have use both in our basic investigations of immunoglobulin production and cell differentiation as well as for passive immunotherapy. Preliminary evidence in another institution has shown that such antibodies may be efficacious in certain types of low-grade B-cell lymphomas.